Most pieces of electronic equipment produced today incorporate one or more circuit boards having different types of electronic components thereon. Each component typically has two or more leads which are inserted through apertures in the circuit board and then solder-bonded thereto. Presently, the trend in electronics is towards reducing the physical dimensions of electronic components to allow for greater component density on a given size circuit board, thus giving rise to a greater number of leads, each connected to a mettalized area on the circuit board. For a typical board, having 50 to 100 components thereon, there may be as many as 5000 component leads each inserted through apertures in the circuit board.
In the past, inspection of the circuit board to determine whether each lead has been inserted through a corresponding aperture and then connected to a corresponding metallized area has been accomplished visually by a human operator. However, as the component density on the circuit board has increased, human operators have experienced greater difficulty in visually inspecting the circuit board for defects. Occasionally, even an experienced operator will fail to detect defects in the form of a missing or misinserted component lead. Moreover, few if any human operators can inspect circuit boards at the same speed at which they are produced.
Because of the difficulties associated with manual inspection of circuit boards, much effort has been devoted to developing machine vision systems for performing this task. One type of vision system now in use for inspecting circuit boards comprises a television camera coupled to an image-processing system. The television camera converts the image of the circuit board into video signals which are processed by the image-processing system to detect defects such as missing or misconnected component leads.
The drawback of machine vision systems which utilize a television camera is that the reduction is limited. Typically, the maximum area that can be viewed on the circuit board by a television camera, while still maintaining sufficient resolution to detect the preesnce of a component lead, is on the order of 2".times.2". For circuit boards having a surface area larger than 2".times.2", the television camera must be moved or stepped across the surface of the circuit board to capture the entire image thereof. Depending on the size of the circuit board, the time consumed in moving the television camera thereacross can be significant, as long as 1 to 2 minutes. As a result, the circuit board throughput of such inspection systems is limited.
Inspection of circuit boards can also be accomplished by vision systems which incorporate a linescan vision camera in place of a television camera. In contrast to a television camera which utilizes a light pick-up device capable of resolving a large two-dimensional image, the linescan camera comprises a plurality of charge-coupled devices, each capable of resolving a very small two-dimensional image. Typically, the linescan camera has 2048 such charge-coupled devices arranged in a linear, one-dimensional array at the image plane of a lens. Depending on the optical characteristics of the lens, each charge-coupled device can typically have a field of view as small as 0.004".times.0.004".
Each charge-coupled device within the linescan camera produces an analog signal varying in intensity with the image within the field of view thereof. Thus, the linear array of charge-coupled devices within the linscan camera collectively serves to capture the image of a thin strip of surface area (e.g., 0.004" wide) running across the circuit board. By moving (scanning) the linescan camera across the circuit board, the image of each of a plurality of successive strips of area on the surface thereof can be captured. Depending on the dimensions of the circuit board, only a single pass of the linescan camera across the circuit board along a single axis thereof is all that is required in order to capture the entire image thereof.
In practice, the analog signal from each of the charge-coupled devices in the linescan camera is output in serial fashion on one or more signal lines. Each analog signal is converted to a digital signal for ease of processing. For each successive strip on the surface of the circuit board within the field of view of the linescan camera, there will be 2048 such digital signals. As may be appreciated, during the scanning of the circuit board by the linescan camera, a large number of digital signals are thus generated.
There are generally two techniques for handling the large number of digital signals arising during the scanning of the circuit board by the linescan camera. The first technique is to process these digital signals immediately after they are generated (i.e., process them "on-the-fly"), storing only a very small number of digital signals associated with the image of one or more strips of area on the surface of the circuit board. The processing of these signals is done by a dedicated high-speed processor which utilizes a particular algorithm designed to inspect for a specific kind of defect, i.e., missing component leads. The flexibility afforded by this technique is extremely limited because the handling and processing of input data from the linescan camera is tied to inspection for the specific defect. To inspect for another kind of defect usually requires that all of the algorithms be changed, which may require extensive reprogramming of the image-processing system.
The other technique for handling the digital signals arising during scanning of the circuit board by the linescan camera is to store all the data in a memory for subsequent processing. Since 1 byte (8 bits) of memory is required to store the image of each 0.004".times.0.004" area on the circuit board, nearly 4 megabytes of memory would be required to store the digital signals representing the image of an 8".times.8" circuit board. The advantage of this technique is that since the image data are stored in memory, the data can be analyzed for different kinds of defects without necessarily changing the manner in which incoming data are handled. However, even though the cost of memory devices has recently dropped, storing all of the digital signals produced during scanning of the circuit board by the linescan camera is generally not as desirable as processing the signals "on-the-fly." Storing the output data of the linescan camera before processing lengthens the inspection process.
Accordingly, there is a need for a technique for rapidly and efficiently inspecting a surface on a substrate, such as a circuit board, for defects thereon using a linescan camera.